1. Field of the Invention.
The invention relates to the field of integrated circuit resistors.
2. Prior Art
In the fabrication of integrated circuits, resistors are often needed. In some cases materials having relatively high resistance such as polysilicon are used. Active devices having low conductivity are also used for resistors. In the fabrication of metal-oxide-semiconductor (MOS) static, random-access memories (RAMs), polysilicon resistors are often employed as loads in the bistable (flip-flop) memory cells.
In U.S. Pat. No. 4,178,674, a process is described for forming a contact region between layers of polysilicon with an integral polysilicon resistor. This resistor includes a resistor region with low resistance areas connected to the region. The resistor is formed by first doping a strip of polysilicon to a first concentration level, this level providing the desired resistivity for the resistor region. Then a masking member is placed over the resistor region and the polysilicon is doped to a second higher level to increase its conductivity to form low resistance regions (leads) on opposite sides of the resistor region.
In the formation of resistors, such as the one described in U.S. Pat. No. 4,178,674, it is desirable to reduce the resistance associated with conductive regions (leads) connected to the resistor region. With a low resistance in these regions, the circuit can operate at a faster rate. The conductive regions are heavily doped (e.g., with phosphorus or arsenic) to reduce the resistance in some cases. However, these dopants have a relatively long diffusion length in polysilicon (approximately 6.5 microns for phosphorus). If a resistor region of approximately 5 microns in length is required, the total length of the polysilicon must be at least approximately 18 microns because of the lateral diffusing of the dopant into the resistor regions from the conductive regions. Thus, considerable lengths are required for these polysilicon resistors when considered in terms of high density integrated circuits.
As will be seen, the present invention provides a resistor and method of fabrication which greatly reduces the total size of the resistor with its conductive regions. High doping of the polysilicon is not used to reduce the resistance of the conductive regions. For this reason, the problems associated with the lateral diffusion from the conductive regions are greatly reduced.